Is "Thanks much for your help." good grammar? I'm a native E speaker and it sounds wrong to me, but I can't justify why it doesn't work. Any help would be much appreciated.

Thanks very much,
M

Marsha,
this is because thanks is a noun. It would be like saying "hugs much". Let me explain:
Verb: I thank you
Verb: I hug you
In these examples, the words "thank" and "hug" are being used as verbs. As they are in these examples:
Verb: He thanks you
Verb: He hugs you
Now, the word "much" in your example is being used like an adverb, and I would expect it to modify a verb. However, it appears to be modifying the noun "thanks".
Noun: Thanks for you
Noun: Hugs for the dog
If you wanted to modify the noun "thanks", it would be usual to use "many" as an adjective. i.e. many thanks for your help.
I hope I have enlightened you a tad. In short, you are not using "thanks" as a verb but as a noun. Therefore, you should use an adjective and not an adverb.

And, please excuse my sentence which starts "As they are"........vooops!

I don't think "thanks" is a noun in this case.

Yes, we give "thanks." In that sense, it's a noun.

Much is an adverb modifying the verb. I think it's fine. More often, however, you hear "Much thanks . . ."

If you think "Thanks much for your help" is fine, then we'll have to agree to disagree, I'm afraid.
I stand by the earlier explanation.

Rgds

What about "many thanks". Would that work?